Genetic and chemoecological studies on sympatric host shift and host race formation in monophagous sawflies

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Mechanisms of the sympatric host shift and host race formation in the field populations of Hemitaxonus japonicus complex were investigated. Main results are as follows:1. The new host race, polyblepharum race is being formed from the original one, tripteron race, in the area of about 10 km wide around 34ﾟ N in the south-western part of Japan without geographical isolation. 2. The adult host selection of two races may be controlled by a polymorphic gene. Their oviposition stimulants are similar but distinct chemical substants, like palmitic acid methyl. 3. Two host races show the very small diferentiation in allozyme polymorphisms and highly repeated DNA sequences. This suggests that the rate of sympatric host race formation is very fast in this sawfly. 4. Some isolating factors such as assortative mating on host plants, asymmetrical mating, larval mortality due to conditioning play important roles for sympatric host race formation. The asymmetrical mating prevents the gene flow from the original host race population to the new one, where the sex pheromones are concerned but not identified yet. 5. We tried to make a new host race in the laboratory. The tripteron race females were crossed to the polyblepharum race males, and the F1 females were mated again with the polyblepharum race males. We have succeeded to get F2 females ovipositing only on Polysticum polyblepharum.